1. Industrial Field of Application
The present invention relates to a rare earth-iron-nitrogen permanent magnet comprising a interstitially nitrogen compound having a Th.sub.2 Zn.sub.17 crystal structure. The present invention also relates to a rare earth-iron-nitrogen magnet obtained by compression molding a powder of the compound having a specified composition, and densifying the molding to obtain a high density bulk magnet by applying shock compression to the resulting molding to prevent decomposition or denitrification from occurring. The present invention further relates to a process for producing the same.
2. Prior Art
Conventionally known high performance magnets are based on rare earth elements include samarium-cobalt (Sm--Co) magnets and neodymium-iron-boron (Nd--Fe--B) magnets. The former type magnets have excellent thermal stability and corrosion resistance, whereas the latter type magnets, which can be produced from low cost starting materials, have extremely high magnetic properties. Hence, both types of magnets are widely used at present.
However, rare earth magnets having further improved thermal stability and high magnetic properties and yet reduced in material cost are still desired from applications such as actuators of electric and electronic parts of motor cars as well as of various types of factory automation machines, and magnets of rotators.
A novel magnet material which satisfy the above demands has been reported recently by J. M. D. Coey and H. Sun, J. Magn. Magn. Mater., 87 (1990) L251, and in JP-A-2-57663 (the term "JP-A-" as referred herein signifies "an unexamined published Japanese patent application"). The disclosed process comprises producing a fine powder of an iron-rare earth compound having a Th.sub.2 Zn.sub.17 crystal structure and allowing the fine powder to react with N.sub.2 gas, a mixed gas of NH.sub.3 and H.sub.2, etc., at a relatively low temperature in the range of from 400.degree. to 600.degree. C. In this manner, a Th.sub.2 Zn.sub.17 type compound containing N atoms intruded into interlattice sites and thereby yielding considerably improved Curie temperature and magnetic anisotropy can be obtained. The compound is thus considered promising as a novel magnet material satisfying the above needs, and its practical use is expected.
The aforementioned Th.sub.2 Zn.sub.17 type compound (referred to as "2-17 system R--Fe--N compound" hereinafter) containing nitrogen atoms in interlattice sites is obtained only as a powder, and it decomposes under an ordinary pressure into .alpha.-Fe and a rare earth nitride at temperatures not lower than about 600.degree. C. It is therefore impossible to obtain a bulk magnet by an ordinary industrial process based on autogeneous sintering based on a diffusion mechanism.
Accordingly, the use of the compound as a bonded magnet using a resin or a low melting metal has been studied. This application, however, has limits in increasing the content of the 2-17 system R--Fe--N compound powder. That is, from the viewpoint of life of the mold and the like, the maximum allowable content of the 2-17 system R--Fe--N compound powder is about 80% by volume. The 2-17 system R--Fe--N compound in the resulting bonded magnet then fails to fully exhibit its superiority in magnetic properties, and falls far behind the conventional Sm--Co system or Nd--Fe--B system magnets concerning the magnetic characteristics. Moreover, the superior magnetic properties and thermal stability of the 2-17 system R--Fe--N compound cannot be fully recognized because of the poor heat resistance of the binder.
An object of the present invention is to provide a densified high performance rare earth-iron-nitrogen permanent magnet from a 2-17 system R--Fe--N compound powder by a process not based on autogeneous sintering, and from which a binder can be omitted. Another object of the present invention is to provide a process for producing the same.